Most likely they're using a rollout in phases based on increasing percentages. In other words, at first only 1% of users get the new page design, then 5%, then 15%, etc..
This is done so if anything is broken, it can be caught before it impacts business to all customers. This is done through the use of cookies. Technically, if you used a cookie editor and knew what to edit, you could force either the old or new design.
Looks snazzy!